In US patent publication 2903860 is disclosed a fuel storage and distribution system for a gas-fuelled sea-going vessel, comprising a gas tank for storing gas fuel, a major portion of which is in liquefied form; a tank room that constitutes a gastight space enclosing tank connections to and from the tank room and valves associated with them; a part of a refrigeration or air conditioning circuit reaching into said tank room, a first local heat transfer circuit in the tank room, which first local heat transfer circuit is configured to receive heat from said part of the refrigeration or air conditioning circuit in said tank room. In the US patent publication 2903860 is also disclosed a method for transferring heat from a heating, ventilation, and air conditioning system of a gas-fuelled sea-going vessel to gas fuel of said vessel, comprising transferring heat from a refrigeration or air conditioning circuit, which reaches into a tank room, to a first local heat transfer circuit in said tank room, and using said first local heat transfer circuit to heat liquefied gas fuel handled in said fuel storage and distribution system.
Natural gas, or in general mixtures of hydrocarbons that are volatile enough to make the mixture appear in gaseous form in room temperature, constitutes an advantageous alternative to fuel oil as the fuel of internal combustion engines. In sea-going vessels that use natural gas as fuel, the natural gas is typically stored onboard in liquid form, giving rise to the commonly used acronym LNG (Liquefied Natural Gas). Natural gas can be kept in liquid form by maintaining its temperature below a boiling point, which is approximately −162 degrees centigrade (−260 degrees Fahrenheit). Natural gas can be also stored for use as fuel by keeping it compressed to a sufficiently high pressure, in which case the acronym CNG (Compressed Natural Gas) is used. This description refers mainly to LNG because liquefying is considered more economical than compressing at the time of writing this text.
FIG. 1 illustrates schematically the architecture of a known system onboard an LNG-fuelled vessel. An LNG bunkering station 101 is located on the deck and used to fill up the system with LNG. The LNG fuel storage system comprises one or more thermally insulated gas tanks 102 for storing the LNG in liquid form, and the so-called tank room 103 where the LNG is controllably evaporated and its distribution to the engine(s) is arranged. Evaporation means a phase change from liquid to gaseous phase, for which reason all subsequent stages should leave the L for liquefied out of the acronym and use only NG (Natural Gas) instead.
The engine 104 or engines of the vessel are located in an engine room 105. Each engine has its respective engine-specific fuel input subsystem 106, which in the case of gaseous fuel is in some sources referred to as the GVU (Gas Valve Unit). The tank room 103 of FIG. 1 comprises two evaporators, of which the first evaporator 107 is the so-called PBU (Pressure Build-Up) evaporator used to maintain a sufficient pressure inside the gas tank 102. Hydrostatic pressure at the inlet of a main supply line 108 inside the gas tank 102 is the driving force that makes the LNG flow into the second evaporator 109, which is the MGE or Main Gas Evaporator from which the fuel is distributed in gaseous form towards the engines. In order to ensure that evaporated gas flows to the GVU(s) and further to the engine(s) at sufficiently high pressure, the PBU system maintains the internal pressure of the gas tank 102 at or close to a predetermined value, which is typically between 5 and 10 bars.
The engine 104 comprises one or more cooling circuits. Schematically shown in FIG. 1 is an external loop 110 of the so-called low temperature (LT) cooling circuit, which may be used for example to cool lubricating oil. The so-called LT water that circulates in the external loop 110 may have a temperature around 50 degrees centigrade when it goes through a heat exchanger 111, in which it donates heat to a mixture of glycol and water that in turn transfers heat to the evaporators 107 and 109. The glycol/water mixture circuit comprises a circulation pump 112 and an expansion tank 113. Glycol is needed in the mixture to prevent it from freezing when it comes into contact with the extremely cold LNG inlet parts of the evaporators 107 and 109.
Many types of sea-going vessels, in particular passenger cruisers, use considerable amounts of energy in various cooling functions, for example to provide air conditioning and to refrigerate food supplies. A prior art document U.S. Pat. No. 8,043,136 suggests using the gas fuel evaporation system to absorb heat from the HVAC system of the vessel. FIG. 2 is a schematic illustration of the heat flows and control functions as taught by FIG. 2 of said prior art document. The core of the prior art system is a heat transfer circuit 201, which absorbs heat from the HVAC system 202 according to arrow 203. The heat transfer circuit 201 donates heat to the gas fuel in a gas fuel evaporation arrangement 204, which in said prior art document is a heat exchanger and/or evaporator through which the gas fuel flows. A control entity 205 monitors the sufficiency of the heat transfer from the HVAC system 202 and augments it, if necessary, by extracting additional heat from sea water 206 according to arrow 207. Another control entity 208 is implemented as a part of the HVAC system 202, so that if not enough cooling takes place by donating heat to the heat transfer circuit 201, electrically driven cooling arrangements can be used to dump heat to the environment 209 according to arrow 210.
Prior art arrangements leave room for improvement in the overall energy efficiency of handling the heat flows on board a gas-fuelled sea-going vessel. Additionally they often include relatively complicated structures and a number of relatively expensive equipment. For example the system of U.S. Pat. No. 8,043,136 requires a pump to circulate the fluid in the heat transfer circuit and another pump to circulate the heat transfer medium in the HVAC circuit, and a total of at least four different heat exchangers. Maritime classification requirements typically require doubling the pumps to achieve reliability through redundancy, which doubles all pump-related costs. Complicated structures mean longer construction times at the shipyard.